.TH std::expint,std::expintf,std::expintl 3 "2024.06.10" "http://cppreference.com" "C++ Standard Libary"
.SH NAME
std::expint,std::expintf,std::expintl \- std::expint,std::expintf,std::expintl

.SH Synopsis
   double      expint( double arg );

   double      expint( float arg );
   double      expint( long double arg );  \fB(1)\fP
   float       expintf( float arg );

   long double expintl( long double arg );
   double      expint( IntegralType arg ); \fB(2)\fP

   1) Computes the exponential integral of arg.
   2) A set of overloads or a function template accepting an argument of any integral
   type. Equivalent to \fB(1)\fP after casting the argument to double.

   As all special functions, expint is only guaranteed to be available in <cmath> if
   __STDCPP_MATH_SPEC_FUNCS__ is defined by the implementation to a value at least
   201003L and if the user defines __STDCPP_WANT_MATH_SPEC_FUNCS__ before including any
   standard library headers.

.SH Parameters

   arg - value of a floating-point or Integral type

.SH Return value

   If no errors occur, value of the exponential integral of arg, that is -∫∞
   -arg

   e^-t
   t

   dt, is returned.

.SH Error handling

   Errors may be reported as specified in math_errhandling.

     * If the argument is NaN, NaN is returned and domain error is not reported.
     * If the argument is ±0, -∞ is returned.

.SH Notes

   Implementations that do not support TR 29124 but support TR 19768, provide this
   function in the header tr1/cmath and namespace std::tr1.

   An implementation of this function is also available in boost.math.

.SH Example

   (works as shown with gcc 6.0)


// Run this code

 #define __STDCPP_WANT_MATH_SPEC_FUNCS__ 1
 #include <cmath>
 #include <iostream>

 int main()
 {
     std::cout << "Ei(0) = " << std::expint(0) << '\\n'
               << "Ei(1) = " << std::expint(1) << '\\n'
               << "Gompetz constant = " << -std::exp(1) * std::expint(-1) << '\\n';
 }

.SH Output:

 Ei\fB(0)\fP = -inf
 Ei\fB(1)\fP = 1.89512
 Gompetz constant = 0.596347

.SH External links

   Weisstein, Eric W. "Exponential Integral." From MathWorld--A Wolfram Web Resource.
